1. Field of the Invention
The present invention relates to the improvement of high-speed search and cue/review functions of a digital audio tape recorder using a rotary head (hereinunder referred to as "R-DAT").
2. Description of the Related Art
With the progress of digital technique, digital recording has come to be adopted for recording various signals. In the field of sound signal recording, compact disks and the like which adopt digital recording have become widespread.
Among these apparatuses which utilize digital recording, digital audio tape recorders (DAT) which are capable of not only reproducing but also recording a sound signal have attracted attention.
DAT's are advantageous in that since a sound signal is recorded on a magnetic tape as digital data, they are free from the problems such as wow flutter, hiss noise and modulation noise, which are inevitable in analog recording, and in that since they have a wide dynamic range which can realize flat frequency characteristics in a wide frequency, recording and reproduction with a high sound quality is realized.
There are two types of DAT's. One utilizes a rotary head, and the other utilizes a fixed head. DAT's using a rotary head (R-DAT) have been standardized and increasingly produced as manufactured goods.
In an R-DAT, a signal is stored in each track inclined at a little over 6.degree. with respect to the plane in which the magnetic tape travels, as shown in FIG. 1.
Each track is separated into an area for recording sub codes consisting of various kinds of information necessary for reproduction, an area for recording an ATF signal for tracking, etc. as well as an area for recording digital sound data (PCM).
The plurality of areas which constitute a track are composed of a plurality of blocks each consisting of 8 bits of SYNC data for synchronizing data, 8 bits of ID code data W1 for designating a frame address, 8 bits of block address data W2 for identifying a block, 8 bits of parity P (simple parity, P=W1+W2) and data of 32 symbols (since 1 symbol consists of 8 bits, the data consists of 256 bits in total). The SUB area is composed of two 8 blocks and the PCM area is composed of 128 blocks.
The R-DAT adopts an interleave format for recording data in dispersion in two tracks in order to suppress the influence of a continuous data error, namely, a burst error to the minimum. In the interleave format, the data of even numbers of an L channel and the odd numbers of an R channel are recorded in one track of the two tracks having different azimuths and the data of even numbers of the R channel and the odd numbers of the L channel are recorded in the other track. By reproducing the L channel and the R channel from the pair of two tracks, it is possible to reproduce sound without break even if an error is produced on either of the two magnetic heads of the R-DAT.
The rotary head is provided with two magnetic heads for tracing tracks of the magnetic tape and two tracks are traced in one revolution of the rotary head. The magnetic tape is so designed as to come into contact with the rotary head which rotates at a high speed only in the range of 90.degree..
One of the important functions of the R-DAT is a high-speed search function. The high-speed search function is a function of detecting the beginning of a piece of music and cueing by causing the magnetic tape to travel at a speed about 200 times as high as that in the normal reproduction.
In such high-speed search, signals are searched at a very high speed such as a speed about 200 times as high as that in the normal reproduction, but the travelling speed of the magnetic tape itself is not so high.
In the R-DAT, the magnetic tape travelling speed in the normal reproduction mode is set at 8.15 mm/s, which is considerably low in comparison with 4.76 cm/s in a conventional analog cassette tape recorder. Since the magnetic tape travelling speed in the FF/REW (forward/rewind) mode is 30 times the speed in the normal mode in a conventional analog cassette tape recorder, if the magnetic tape is caused to travel at a similar same speed in the R-DAT, it is possible to cause the magnetic tape to travel at a speed about 200 times as high as that in the normal reproduction mode of the analog cassette tape recorder.
It is necessary to read out data indicating the beginning of the piece of music from the magnetic tape which is travelling at such a high speed.
In the R-DAT, the recording area on the magnetic tape is provided with a sub code area in which ID (S-ID) indicating the beginning of the music, and data W1, W2 indicating the program number (PNO) are recorded. These data are recorded in multiples in a plurality of blocks (8 blocks in both sub code areas 1 and 2) in order to enhance the accuracy of reading the data.
Parity data are attached to the data W1 and W2 in order to prevent an error in reading. In a conventional apparatus, the parity of the data W1, W2 is first checked and if the parity is identified, this pair of items of data is fetched. However, since the probability of causing an error is large if only one pair of items of data W1, W2 is fetched, it is not until two pairs of items of same data W1 and W2 are serially fetched that cue up or the like is executed.
High-speed search is classified into FF search (forward fast feeding) and REW search (rewinding) by the direction of travel of the tape. The direction of rotation of the head is the same in high-speed search at 200 times the speed as in the normal mode. In order to read data by a reading signal having the same frequency as in the normal mode, it is necessary to make the relative speed of the magnetic head and the tape surface toward the track the same.
According to a simple calculation, in the case of high-speed search at 200 times the speed in the normal mode, the head rotation speed, which is 2,000 rpm in the normal mode, must be set at about 3,000 rpm for FF search and about 1,000 rpm for REW search.
In this way, in high-speed search, the head rotation rate is varied between FF search and REW search. Therefore, the time required for the head to pass the sub area in the track for FF search is about 1/3 of the time for REW search. As shown in FIG. 4A, the trajectory of the head at the time of FF search is formed in the direction in which the track is crossed.
For this reason, the probability of accurate reading of data is sometimes greatly reduced at the time of FF search and the high-speed search function cannot be fully exhibited.
In addition, the R-DAT having such a high-speed search function is often inconvenient if a magnetic tape on which the start ID, which is sub code data indicating the beginning of a piece of music, is not recorded. As a counter measure, an R-DAT having a cue/review function which enables cue up while listening to reproduced sound has been developed.
In cue/review reproduction in a conventional R-DAT, however, since one item of the digital data read out of the two magnetic heads is adopted, an exterior circuit such as an envelope circuit for comparing the amplitudes is necessary, which disadvantageously makes the system structure complicated.
FIGS. 5A to 5F show the tracks which the two magnetic heads A, B (having a + azimuth and - azimuth, respectively) trace at the time of cue/review and the envelope waveforms of the signals reproduced at that time. In the case of reproducing the signals recorded in the tracks shown in FIG. 5A in the normal mode (.times.1PB) by a head switching signal, since the magnetic heads A, B trace the +azimuth tracks and the - azimuth tracks so as to read digital data, as shown in FIG. 5B, in the envelope waveform of the reproduced signals, the amplitudes of the signals from the magnetic heads A, B are approximately the same, as shown in FIG. 5E. On the other hand, at the time of reproduction at double the speed (.times. 2PB), the magnetic head A traces the + azimuth tracks and the magnetic head B also traces the + azimuth tracks, as shown in FIG. 5C. Since it is impossible to read out digital data by the tracing of the +azimuth tracks by the magnetic head B, in the envelope waveform, the amplitude of the signal from the magnetic head A is large, while that from the magnetic head B is small. It is therefore necessary to select only the digital data from the magnetic head A by the envelope circuit for reproduction, which leads to a complicated system structure.